1. Field of the Invention
Embodiments of the invention relate generally to tomographic reconstruction of an object and, more particularly, to medical imaging by tomographic reconstruction or tomosynthesis with a small number of projections (“few view tomography”).
2. Description of Related Art
The acquisition of 1D or 2D projection images of an object and the tomographic reconstruction of an image having a greater dimension, either 2D or 3D, of this object are schematically illustrated in FIG. 1.
The reconstruction of an image by tomography consists of emitting X-rays 10 from a source towards the object 12, the X-rays are emitted according to different angulations l ε {1, . . . , L} which define the trajectory Tr of the source (commonly a rotation, also called “spin”). The 3D reconstruction of the object is described below.
After having crossed the object 12, the X-rays are detected by a detector 13 so as to form a set of 2D projections. There are as many acquired 2D projections as there are relevant angulations (i.e. L projections for the trajectory). The acquisition is applied by a detector 13 located facing the X-ray source 11, for example, a digital camera. It is possible to apply the acquisition with a fixed detector and a source, which is not necessarily positioned facing the source.
An application of tomography is the detection and characterization of a lesion in an organ, for example, a breast cancer tumour or stenosis in a vessel of a patient. The acquired 2D projections are used for reconstructing a 3D image of the object. This 3D object is more specifically a 3D mapping of X-ray attenuation coefficients of the crossed medium. It is by means of this mapping that the radiology practitioner interprets this image according to the observed contrast differences.
The commonly reconstructed 3D images (for example by an algorithm of the filtered back-projection family) are affected with striations (streaks) due to the finite sampling of the object of interest. Each reconstructed point of the object is the origin of a bundle of lower intensity streaks but proportional to the intensity of the object in the relevant point and to the width of the angular pitch for which no measurement is available. The streaks, therefore, disappear when the angular pitch tends toward zero, i.e. for a large number of projections covering at least 180°. When the number of projections is limited by the rate of the imaging apparatus or does not cover 180°, because certain angulations are not accessible, the image is altered by sub-sampling streaks.
An problem is that the object to be reconstructed may include strong contrast differences. Thus, the streaks issued from more intense structures may strongly degrade the less intense structures or the lower contrast differences.
This phenomenon is all the more significant in the medical field when the imaged objects are organs crossed by a contrast product. This is also the case when the imaged object is a breast in which microcalcifications are found.